A method of this kind is known from WARP, R. J.; DOBBINS, J. T.: Quantitative evaluation of noise reduction strategies in dual-energy imaging, in: Med. Phys. 30 (2), pages 190 to 198, February 2003. In the context of dual energy X-ray absorptiometry, X-ray beams with differing energy levels are directed at the examination subject, generally a patient. Dual energy X-ray absorptiometry can be accomplished using a single X-ray shot or a series of consecutively taken shots.
In the first case, a dual detector is used which incorporates two different scintillation materials whose response has energy centers that are as far apart as possible. In the second case, consecutive shots are taken using as differing X-ray spectra as possible which, when using X-ray tubes, can be generated by varying the tube voltage by means of which the electrons are accelerated, or by selecting pre-filters.
For each pixel of the projection images recorded, the material composition in the beam path between the point X-ray source and the pixels can be inferred from the attenuation behavior at the different energy levels. The projection images will hereinafter also referred to as attenuation images. Material composition is also to be understood as the mass density of the different materials along the beam through the examination subject.
By combining the attenuation images captured at different energy levels, mass density images can be created which render, at least approximately, the mass density of the materials contained in the subject examined. The attenuation images are usually combined linearly, the weighting factors being determined empirically. However, mathematically precise determination of the mass density is virtually impossible using the known methods.
Mathematically precise determination of mass density is also made more difficult by the scattered radiation present. Even in projection radiography using flat panel detectors, scattered radiation plays a critical role because of the large solid angle detected. To reduce the scattered radiation, anti-scatter grids are frequently inserted directly above the detector input surface. As a quantitative method, dual energy X-ray absorptiometry places more exacting requirements again on the accuracy of the measurement data than simple projection imaging in the context of projection radiography. In spite of anti-scatter grids, the data-distorting scatter fraction may be considerable. For example, in the thorax area a very small air gap is usually employed, with the result that in spite of anti-scatter grids the scatter intensity may if anything outweigh the primary intensity, particularly in image regions with high attenuation and at higher photon energies, corresponding to X-ray tube voltages above 100 kV. Moreover, it is an empirical fact that scatter fractions are very different for the higher and lower energy image data, particularly in the case of a small air gap, i.e. where the distance between the scatter object and the detector is small. All in all, in dual energy X-ray absorptiometry the presence of scattered radiation may, in spite of anti-scatter grids, yield unreliable and in some cases unusable results, e.g. negative material thicknesses. Scatter correction is therefore of major importance for dual energy X-ray absorptiometry.
With dual energy X-ray absorptiometry, it is therefore necessary to use computerized scatter correction methods in addition to anti-scatter grids.
It should be noted at this point that scattered radiation will hereinafter also be referred to as secondary radiation. The sum of primary and secondary radiation which produces the measured image values will be termed total radiation.
HINSHAW, D. A.; DOBBINS III, J. T.: Recent progress in noise reduction and scatter correction in dual-energy imaging. In: Proc. SPIE, 1995, Vol. 2432, pages 134 to 142, describe a scatter correction method in the context of dual energy X-ray absorptiometry. In the known method, for each of the attenuation images captured at different energy levels, scatter correction is performed by determining for a given pixel an empirically calculated scatter fraction as a function of the pixel. The scatter fraction determines the shape and width of a distribution function for the scattered radiation. On the basis of the scatter function, the scatter contributions in adjacent pixels are calculated. The method is then repeated for further image values and the scatter contributions in the individual pixels are summed. A convolution of the image registered by the detector device with a distribution function therefore takes place, the width and shape of which depend on the image values of the attenuation image captured by the detector device.
FLOYD, C. B.; BAKER, J. A.; LO, J. Y.; RAVIN, C. E: Posterior Beam-Stop Method for Scatter Fraction Measurement in Digital Radiography. In: Investigative Radiology Feb. 1992, Vol. 27, pages 119 to 123, describe a measurement-based method for determining scattered radiation according to the beam-stop method. This method is suitable for laboratory applications using phantoms, but hardly for clinical use.
In addition, ZELLERHOFF, M.; SCHOLZ, B.; RÜHRNSCHOPF, E.-P.; BRUNNER, T.: Low contrast 3D-reconstruction from C-arm data. In: Proceedings of SPIE. Medical Imaging, 2005, Vol. 5745, pages 646 to 655, describe various computerized methods for scatter correction in the context of computed tomography.
However, the known computerized methods are generally quite complex and laborious.
Moreover, there therefore exists a need for comparatively simple correction methods enabling significant image quality improvements to be achieved.